Proxying
Overview
The Backstage backend comes packaged with a basic HTTP proxy, that can aid in reaching backend service APIs from frontend plugin code. See Call Existing API for a description of when the proxy can be the best choice for communicating with an API.
Getting Started
The plugin is already added to a default Backstage project.
In packages/backend/src/index.ts
:
const proxyEnv = useHotMemoize(module, () => createEnv('proxy'));
const service = createServiceBuilder(module)
.loadConfig(configReader)
/** ... other routers ... */
.addRouter('/proxy', await proxy(proxyEnv));
Configuration
Configuration for the proxy plugin lives under a proxy
root key of your
app-config.yaml
file.
Example:
# in app-config.yaml
proxy:
endpoints:
/simple-example: http://simple.example.com:8080
'/larger-example/v1':
target: http://larger.example.com:8080/svc.v1
headers:
Authorization: ${EXAMPLE_AUTH_HEADER}
# ...or interpolating a value into part of a string,
# Authorization: Bearer ${EXAMPLE_AUTH_TOKEN}
Each key under the proxy configuration entry is a route to match, below the
prefix that the proxy plugin is mounted on. If it does not start with a slash,
one will be prefixed automatically. For example, if the backend mounts the proxy
plugin as /proxy
, the above configuration will lead to the proxy acting on
backend requests to /api/proxy/simple-example/...
and
/api/proxy/larger-example/v1/...
.
The value inside each route is either a simple URL string, or an object on the format accepted by http-proxy-middleware.
If the value is a string, it is assumed to correspond to:
target: <the string>
changeOrigin: true
pathRewrite:
'^<url prefix><the string>/': '/'
When the target is an object, it is given verbatim to http-proxy-middleware
except with the following caveats for convenience:
- If
changeOrigin
is not specified, it is set totrue
. This is the most commonly useful value. - If
pathRewrite
is not specified, it is set to a single rewrite that removes the entire prefix and route. In the above example, a rewrite of'^/api/proxy/larger-example/v1/': '/'
is added. That means that a request to/api/proxy/larger-example/v1/some/path
will be translated to a request tohttp://larger.example.com:8080/svc.v1/some/path
.
There are also additional settings:
allowedMethods
: Limit the forwarded HTTP methods. For exampleallowedMethods: ['GET']
enforces read-only access.allowedHeaders
: A list of headers that should be forwarded to and received from the target.
By default, the proxy will only forward safe HTTP request headers to the target.
Those are based on the headers that are considered safe for CORS and includes
headers like content-type
or last-modified
, as well as all headers that are
set by the proxy. If the proxy should forward other headers like
authorization
, this must be enabled by the allowedHeaders
config, for
example allowedHeaders: ['Authorization']
. This should help to not
accidentally forward confidential headers (cookie
, X-Auth-Request-User
) to
third-parties.
The same logic applies to headers that are sent from the target back to the frontend.